Autosomal dominant polycystic kidney disease is one of the most common lethal genetic diseases, affecting one person in a thousand. Mutations in the PKD1 gene cause the majority of cases. The PKD1 gene codes for a large protein of unknown function whose product, polycystin, is predicted to be a receptor. Its large extracellular domain contains 16 copies of a novel motif, the PKD repeat, that is likely to be a ligand binding domain based on structural homology to immunoglobulin repeats. The goal of this project is to identify polycystin ligands. During kidney development, polycystin is highly expressed in the branching epithelium of the ureteric bud. We have developed a novel bioassay for the identification of ligand binding domains in polycystin. Using this bioassay, we have implicated a conserved amino acid sequence from the PKD repeats of polycystin in the patterning of the ureteric bud epithelium during morphogenesis. Ureteric bud branching is affected in PKD1 knockout mice and in tissue culture model systems of branching morphogenesis using PKD1 null cells. We have isolated two polypeptides, p49 and p53, from extracts of fetal murine kidney that bind specifically to GST fusion proteins containing a PKD repeat and to a peptide affinity column containing the critical amino acid sequence of the PKD repeat. These polypeptides are candidate polycystin ligands. The goals of this proposal are to identify and characterize these proteins and to test whether they are indeed polycystin ligands.